1. Field of the Invention
The present invention relates to a composition for a water-based inkjet printer ink for printing images that can be retained on a wide range of porous and non-porous substrates. More particularly, the present invention relates to an improved water-based inkjet printer ink that contains a pigment colorant, that is free of polymeric binders, that can be utilized in a variety of printhead configurations for printing on different substrates, and that enables successful and reliable printhead operation for longer periods of time than water-based inkjet inks containing polymeric binders.
2. Description of the Related Art
Ink-jet printing is a non-impact printing technique in which droplets of ink are ejected through a very small nozzle onto a substrate without bringing the nozzle into contact with the substrate.
There are many demanding performance requirements for dyes and inks used in inkjet printing. For example they desirably provide sharp, non-feathered images having good water and lighffastness, as well as high optical density. The inks are often required to dry quickly when applied to a substrate, in order to prevent smudging, but they should not dry to form a crust over the tip of an ink-jet nozzle, which would stop the printer from working. The inks should also be storage stable over time without decomposing or forming a precipitate that could block the small inkjet nozzle.
The inkjet printing industry can be grouped into two broad market categories—desktop printing on the one hand, and commercial and industrial printing on the other hand. The desktop printing market category is based upon the use of inks that have been optimized primarily for printing documents and photographs. Commercial printing applications range from printing wide format posters, banners, and billboards, to product coding on cans and bottles. For present purposes, the printing of wide format graphics will be considered as commercial graphics. Industrial printing applications include direct mail, addressing, product coding, and marking.
The inks used in those applications have different requirements. In the desktop market, the print droplet must be tightly controlled. Additionally, desktop printers print on limited types of substrates. In commercial and industrial printers, on the other hand, a high premium is often placed on being able to print on a wide range of substrates. While print quality and image opacity are important in industrial markets, they are often of secondary importance. Industrial inkjet printing is an established, expanding market. Inkjet printers are widely used to place images on mail and business forms, on primary and secondary packaging.
Inkjet printers provide an inexpensive, reasonably fast, and relatively high quality image output. Inkjet printing involves withdrawing small quantities of ink from a reservoir, converting the ink into very small drops, and delivering the drops through the air to impact the surface of the substrate to be printed with the desired image.
Inkjet printing falls into two general categories: continuous inkjet printing and drop-on-demand inkjet printing. However, the greater volume of inkjet printing today utilizes a drop-on-demand method, which most often the type of inkjet printing done by home and office inkjet printers connected to personal computers, and it is less often used in commercial and industrial applications. The drop-on-demand category is further subdivided into the thermal inkjet method and the piezoelectric inkjet method.
Most consumer-type inkjet printers employ the thermal method and utilize a print cartridge having a series of tiny, electrically-heated chambers that are formed by photolithography. To produce an image, the printer runs a pulse of current through the heating elements. A steam explosion in the chamber forms a bubble, which propels a droplet of ink through a nozzle and onto the substrate, usually some type of paper. When the bubble condenses, surplus ink is pulled back up from the printing surface. The ink's surface tension pumps another charge of ink into the chamber through a narrow channel attached to an ink reservoir. In recent years, the thermal print-heads are being utilized more and more for industrial applications.
Historically, most commercial and industrial inkjet printers used a piezoelectric crystal in each nozzle instead of a heating element. When current is applied, the crystal bends, forcing a droplet of ink from the nozzle and projects it toward the substrate to be printed. Piezoelectric inkjet printing permits the use of a wider variety of different types of inks than does either thermal or continuous inkjet printing, but it is a more costly process. The emerging inkjet material deposition market uses inkjet technologies, typically piezoelectric inkjet, to deposit materials on substrates
The continuous inkjet method is most often utilized for commercial printing or for applying markings on products and packages. In continuous inkjet technology, a high-pressure pump directs liquid ink from a reservoir through a small nozzle, creating a continuous stream of ink. A piezoelectric crystal causes the stream of liquid to break into droplets of uniform size at regular intervals. As they form, the ink droplets are subjected to an electrostatic field created by a charging electrode. The field is varied according to the degree of drop deflection desired. The result is a controlled, variable electrostatic charge on each droplet. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighboring droplets. In the so-called binary deflection method of continuous inkjet printing, the image is printed by the uncharged drops, while the charged drops are deflected into a catcher and are re-circulated. The primary advantage of that method is that the complex charging schemes needed to compensate for electrostatic and aerodynamic interactions between charged drops are avoided.
In another type of continuous inkjet printing, the so-called multiple deflection method, drops of ink are charged by an electrode located at the point of drop break-off, and are then deflected by high voltage plates to print a dot at a position proportional to the applied charging voltage. A primary advantage of the continuous inkjet method is the greater number of drops per unit time that are available per printing element.
Presently two principal types of inks are used in inkjet printing—water-based inks and solvent-based inks. There is a very small amount of UV-curable ink sold, but it represents an extremely small percentage of the total volume of industrial inks.
Current water-based inks tend to be the popular choice for inkjet printing on porous substrates, such as paper, corrugated board, and some untreated coated board. The inks run reliably, and penetration of the ink into the substrate can be controlled to a large degree. Additionally, water-based inks are environmentally friendly and are often the only types of inks that will run reliably in many different types of inkjet printheads. Although in some instances the permanence of water-based inks on substrates can be improved by the addition of resins or crosslinking polymers, the additional permanence is not deemed sufficient for many applications. More significantly, however, the use of resins or polymers as an inkjet ink component adversely affects the reliability of the printer by causing clogging of the small ink nozzles after a relatively short period of time.
Solvent continuous inks print on a wider range of substrates, and they tend to be used primarily on non-porous substrates. The solvent inks are often made of highly volatile solvents that evaporate and must be replenished. The solvent volatility leads to problems such as an odor that emanates from the printed substrate, and it requires that the printer have sufficient clearance to print with highly flammable, volatile fluids. Also, most solvent-based inkjet printers tend to print relatively crude images when compared with images produced with most water-based inks. There are solvent-based, drop-on-demand printers that can be used for printing on some porous substrates as well as on non-porous substrates.
For many industrial applications, variable deflection continuous ink printers are adequate despite the poor resulting print quality. There exist a wealth of inkjet formulation for these printers, the vast majority of which are based on low boiling point solvents such as MEK (methlyethylketone) and methanol. Those printers are designed to run resin-based inks, so the printing of permanent images on non-porous substrates is established and well known.
Water-based inkjet inks are typically not used to print on semi-porous and non-porous substrates in industrial applications, primarily because of the poorer adhesion of the ink to the substrate surface, often resulting in images that have unsatisfactory rub resistance.
The other type of continuous inkjet technology, binary array inkjet, successfully prints water-fast images only when running solvent inks. The limitations of the water-based ink formulations were one of the driving forces leading to the introduction of a solvent-based continuous inkjet array printer.
prop-on-demand printers have been successful in some industrial applications, but the inks used for those applications tend to be oil based, solvent based, or, more recently, UV-curable inks. In the late 1990's, thermal drop-on-demand inkjet technology was introduced into industrial markets. Initially aimed at mailing and addressing applications, it has more recently been used in packaging applications.
Inks for thermal drop printers tend to be largely composed of water. Although solvent based and UV-curable inks are known to be operable in those types of printers, the reliability of the inks is generally not acceptable in an industrial environment, where the productivity of the printer is especially important.
A further limitation of thermal drop printheads is that it is difficult to print using a sufficient quantity of a resin or polymeric binder to achieve any significant permanence on non-porous substrates, and also to maintain an acceptable level of printhead reliability. Typical problems associated with resins and polymeric binders include clogged jets when running after a period of time, and especially when restarting the printer after it has been idle for a period.
It is an object of the present invention to provide a water-based inkjet ink that can be utilized for printing on both porous and non-porous surfaces, and that also operates reliably on inkjet printers having different types of printheads.